prep for pyopenvdb ( D8123 )
Note: this will only change openvdb to be build as a shared library.
pyopenvdb will not build/link yet as further patches are needed
to make it deal nicely with static boost (unless we want to move
boost dynamic as well)
Not looking to land this until 2.91 (but we currently do not have a 2.91 tag yet) so no rush to review it.